The electric and magnetic components of an electromagnetic field are normally measured by extremely small probes which are connected by a coaxial cable to a measuring device such as a meter, an oscilloscope or other device.
A short linear antenna is used for electric fields and a small wire loop having one or more turns is used to measure magnetic fields. The very presence of the probe or particularly a large metallic cable operates to distort the field. To complicate matters further, the connecting cable can accommodate foreign fields which are directed to the measurement area because of its metallic nature and because it operates as, for example, a pick-up antenna. In regard to this, severe field disturbances occur when one tries to employ a conventional probe to measure fields inside electronic equipment which are caused by external electrostatic discharges.
As one can ascertain, a common design problem involves trying to prevent such discharges from disrupting the delicate electronics inside a reasonably shielded enclosure. Thus, any metallic wire which enters such an enclosure represents a breach through which external fields can enter and fields enter quite easily on a conductive wire. The design problem then often involves special grounding treatment of each wire entering the enclosure. Unfortunately, attempts to measure fields inside such an enclosure compound the problem by coupling more of the undesirable fields to the inside of the enclosure along the probe connection.
For an example of the problem, the following is applicable. In regard to modern telephone communications systems a common requirement is that the telephone equipment be able to withstand electrostatic discharges without damage and with a minimum of disruption. There are many definitive documents which specify that such equipment must be relatively immune to electrostatic discharge.
As one can ascertain from such documents, they define a human body model for discharge and specify various severe levels with 15,000 volts of electrostatic discharge being the highest. Thus a goal has been to design telephone systems which can withstand that level of discharge with no damage and a minimum of logic disruption. As one can ascertain, modern telephone systems are relatively complex and many employ central control logic as well as distributed control logic where microprocessors are widely employed, including extensive logic circuitry which is coupled and operates in conjunction with such microprocessors. The operators console associated with such systems serves as a control for the switching system and contains complicated electronic assemblies.
In regard to such consoles, one can understand that the equipment utilized therein is subject to numerous static discharges, especially when used in carpeted areas. One can also ascertain that normal logic circuitry is very sensitive to these discharges. Hence, one must immunize the logic as well as the housings containing the logic as is apparent. In any event, it is extremely difficult to know what the electric field strength is inside such a housing because logic disruption problems, as indicated above, are caused by wires or cables coupled to the units which act as antennas bringing the external discharge field into the cabinet or the housing. In this manner, any probe wire would have compounded the original problem due to the fact that a probe wire itself acts as an antenna.
There is therefore an obvious need to measure or monitor the fields inside a partially shielded enclosure and once the fields are measured then one, of course, could deal with the problem and provide immunity in regard to that problem. The prior art was of course aware of the fact that such fields had to be measured.
In regard to some of the prior art, reference is made to U.S. Pat. No. 4,542,338 entitled "Optical Fiber Interferometric Electric Current Measuring Device" issued on Sept. 17, 1985 to H. Arditty et al. This patent discloses a device which uses the Faraday magneto optic effect to measure strong electrical or magnetic fields. Such devices are used to measure fields at a remote location while providing isolation. The device employs a winding which is traversed by a feedback current and includes an interferometer core, a feed back winding, an insulating member and a measuring head which are mounted on a conductor which is traversed by the current to be measured. The insulating member is a rigid member forming a support or a tubular insulating member having a flexible wall and provided with skirts made from an insulating material having a profile similar to that of a water droplet.
See also, U.S. Pat. No. 4,516,073 entitled "Magnetometer Probe Using A Thin Film Magnetic Material As A Magneto Optic Sensor" issued on May 7, 1985 to G. Doriath et al. This patent shows a measuring device which is divided into two separate sub units including a first sub unit which has means for transmission and detection of polarized light and which is coupled via a fiber to a second sub unit comprising a thin layer of magnetic material within which is guided the light transmitted by the fiber. The coupling with the fiber is established at the edge with the opposed end of the layer being provided with a mirror so that the guided wave follows an outgoing and returning path before being retransmitted to the detection means which then operates to measure the rotation of the polarization of the light which is caused by the Faraday effect. Essentially this is another device using the Faraday magneto-optic effect to measure fields.
U.S. Pat. No. 4,032,843 entitled "Compensated Signal Isolator" issued on June 28, 1977 to R. S. Loucks. This patent describes a device for coupling a signal value out of a circuit which is floating at some relatively high voltage. There is shown a sensor for converting the parameter to be measured to a signal voltage together with an amplifier and light emitting diode which are powered by a floating power supply of minimum power rating. An optical fiber link, which provides electrical isolation, joins the LED with a solid state light to electric transducer at a remote location. There is shown circuit means responsive to the light to electric transducer for outputting an analog value representative of the quantity to be measured at the point of a common mode voltage. This LED fiber optic link and light to electric transducer constitute a first optically coupled isolator. An additional isolator having the same characteristics as the first is located at the remote location and includes a second LED which is linked to the first via an optical fiber and includes a second light to electric transducer which generates a signal representative only of the extraneous variations produced through the first isolator. The resulting signal is differentially added as a feedback term to the signal produced by the first optical isolator device to provide a compensated analog output signal.
See also, PCT U.S. 86/02218 International Publication No. WO87/02769 entitled "Fiber Optic Sensing of Temperature and/or Other Physical Parameters", published on May 7, 1987 to H. Sun et al. Essentially, this describes probes which utilize direct optical effects for remote sensing of temperature and other parameters. Particularly, a special temperature sensitive luminescent phosphor is coated on the fiber. The phosphor temperature can then be deduced from the emitted wave lengths. As indicated, the probes provide isolation of high voltages and reduction of the dangers associated with discharge, such as explosion, when using wires.
In any event, as one can ascertain from the prior art, there is a definite need to provide a probe which can convey electromagnetic field information across a shielded boundary without using electrically conductive wires.
It is a further object of the present invention to provide apparatus and methods of using an optical signal to carry field strength information which apparatus utilizes interchangeable probe tips for measuring specific types of field disturbances.